The present invention relates generally to centrifugal pumps, and more particularly, to a method and apparatus of detecting torsional disturbances or alternately mechanical disturbances that cause displacement of the motor's rotor in a centrifugal pump assembly using voltage and current data acquired from voltage and current sensors in the pump motor controller assembly.
Submersible types of centrifugal motor pumps are used for a number of applications, such as drinking water supply, irrigation, and de-watering as well as in offshore applications. In these applications and others, the motor as well as the pump may be submerged and installed in deep wells down to several thousand meters. Moreover, motor power can exceed 2,000 kW and voltages over 10,000 V. As a result of the remote location of these pumps, condition monitoring and detection of defects at an early state are often difficult. For example, sensors for shaft vibration often fall or are not practical as they cannot efficiently withstand high ambient water pressure. Additionally, signal cables may be used to translate signals to a surface monitoring device but the cables are often damaged during pump installation to a deep well. As a result, most submersible pumps work with an overload switch as the only protection mechanism. These overload protection devices normally detect overload, underload, or phase differences. As power consumption of the pumps change widely with operation point, the pump protectors have to be adjusted rather insensitively so that small changes in motor current caused, for example, by worn out bearings are not detected.
Mechanical disturbances or interference in motor/centrifugal pump assemblies may be caused by several conditions. For example, severe bearing deterioration may result in binding of deteriorated balls of the bearing or of rubbing in the area between wear rings and the pump rotor. In close-coupled pumps touchdown of a motor rotor to the stator may occur resulting in mechanical disturbances. Shaft misalignment or bent shafts may also create interference through vibration and torque ripple. Debris which may be lodged in or around the pump impeller may also create mechanical interference. Moreover, loose impeller or unstable foundation may also create interference and disrupt proper operation of the pump.
Because of the location of the submersible pump during operation, it is typically difficult to detect the onset of a mechanical disturbance. Some systems have been developed to detect the early onset of a mechanical failure using extra instrumentation or separate modules connected to cables placed in the deep well with the pump. This additional instrumentation, however, adds to the cost of the pump and damage to the cables often occurs when placed in a deep well.
Centrifugal pumps used in process industries such as refineries are often critical to the process. Pump failure may result in severe economic loss due to unscheduled plant shutdown and the attendant cleanup and restart required after unscheduled shutdown. These critical pumps are sometimes fitted with vibration monitoring equipment, or are subject to periodic testing with portable equipment to try to predict developing faults. However, the installation cost of in-place monitoring is high and the skilled labor associated with periodic testing is costly.
It would therefore be desirable to design a pump assembly wherein mechanical disturbances or interferences are quickly identified and detected without additional instrumentation in the pump.